Radio receivers, or tuners, are widely used in applications requiring the reception of electromagnetic energy. Applications can include broadcast receivers such as radio and television, set top boxes for cable television, receivers in local area networks, test and measurement equipment, radar receivers, air traffic control receivers, and microwave communication links among others. Transmission of the electromagnetic energy may be over a transmission line or by electromagnetic radio waves.
The design of a receiver is one of the most complex design tasks in electrical engineering. In the current state of the art, there are many design criteria that must be considered to produce a working radio receiver. Tradeoffs in the design""s performance are often utilized to achieve a given objective. There are a multitude of performance characteristics that must be considered in designing the receiver.
However, certain performance characteristics are common to all receivers. Distortion and noise are two such parameters. The process of capturing the signal creates distortion that must be accounted for in the design of the radio receiver. The distortion must either be filtered out or canceled. Once a radio signal is captured, the noise surrounding the received signal in the receiver must be considered. Radio signals are often extremely weak and if noise is present in the circuit, the signal, even though satisfactorily received, can be easily lost in this noise floor. The current state of the art in receiver design is often directed to overcoming these receiver limitations in a cost effective manner.
In an integrated radio receiver ESD discharge circuitry is typically utilized to protect the integrated circuit from static discharge. Radio signals in a receiver tend to be of small amplitude and high frequency and are therefore susceptible to distortion caused by capacitive loading by standard ESD control methods. It is therefore desirable to provide a system of ESD protection that does not interfere with the reception of the high frequency, small amplitude signals.
Inductors are utilized in a receiver to provide frequency selectivity that helps eliminate distortion and interference. Inductors are not easily integrated onto a semiconductor substrate. Spiral inductors typically used have a low Q that provides insufficient selectivity, requiring filters to be fabricated off of the integrated circuit substrate.
Amplifiers are utilized to boost signal levels above the receiver noise floor. Amplification is used in many receiver functions. It is used in a fixed gain amplifier to provide a fixed gain to a signal presented to it. In providing a fixed gain a signal of a given power level presented to an amplifier is increased in power by a fixed multiplication factor. In a variable gain amplifier (xe2x80x9cVGAxe2x80x9d) gain is often adjusted to provide an output signal of fixed power for a variety of input signal power levels. The multiplication factor is adjusted by a control depending on the power of the input signal.
Amplification is often used in conjunction with other circuit functions. Filters often incorporate amplification to boost a desired signal""s level while simultaneously rejecting unwanted signals. Attenuators also incorporate amplifiers to expand their dynamic range. Thus an attenuator with gain included can produce an output signal having more or less power than a signal input to the device, depending on the setting.
Due to inherent amplifier nonlinearities the amplifiers produce distortion. Distortion tends to vary with the signal level presented to an amplifier. Strong input signals tend to increase distortion levels. Often to limit distortion the dynamic range of an amplifier is constrained to a narrow range of input signal levels to prevent distortion from arising. Constraint on signal level affects a receiver system""s overall performance.
For example, constraint on input levels requires tight automatic gain control (xe2x80x9cAGCxe2x80x9d) of the receiver giving rise to further problems of stability, response time, and maintenance of the required signal level range. Amplifiers with an increased dynamic range are thus desirable in designing receivers to decrease distortion and to relax systems requirements.
There is therefore provided in a present embodiment of the invention, a large gain range, high linearity, low noise MOS VGA. An embodiment of the integrated MOS VGA having improved dynamic range comprises a substrate and a first differential pair amplifier disposed upon the substrate. The first differential pair amplifier is coupled to the VGA output and has a gain that contributes to the VGA gain in direct proportion to the first differential pair amplifier gain.
A second differential pair amplifier disposed upon the substrate is coupled to the VGA output and has a gain. The second differential pair amplifier is coupled to the first differential amplifier such that an increase in the second differential pair amplifier gain contributes in inverse proportion to the VGA gain.
A fixed control current is split between the first differential pair amplifier and the second differential pair amplifier such that the current to the second differential pair amplifier source connection is not greater than the current applied to the first differential pair amplifier source connection and applied such that an increase in current causes an increase in amplifier gain.